The present invention relates to a method and apparatus for annealing closed loop metallic objects in a magnetic field, and more particularly the invention concerns a furnace which allows continuous magnetic field annealing of amorphous metal cores of the type used in electrical transformers.
For some time, it has been recognized that the magnetic core of an electrical transformer may be fabricated from coiled strips of a glass-like metal alloy having a random non-crystalline atomic structure. Such cores, which are commonly referred to as amorphous metal cores, have several advantages as compared to the standard metal cores. For example, a transformer with an amorphous metal core has a no load power loss which is substantially lower than that of a transformer with a standard metal core.
It is known in the art to anneal amorphous metal cores in a furnace under controlled heating and cooling cycles, while concurrently applying a magnetic field to the cores. The magnetic field is generated by passing direct electrical current through a conductor that is positioned in the central opening of the core, during the annealing process.
Present magnetic field annealing furnaces operate as batch processes. In the batch process, each core within a batch of cores is individually loaded onto specially designed carts that carry conductive magnetizing shafts that are vertically disposed. The cores are loaded horizontally so that the vertical shafts are passed through the central opening of each core. The shafts are series connected to a current source such that each core is subjected to its own magnetic field and the batch is placed into the furnace where the lengthy heating and cooling cycles begin. After the annealing is complete, the batch must be individually unloaded so a new batch can be annealed. As will be apparent, the batch process as described above is relatively slow and labor intensive.
A continuous process for magnetic field annealing has been proposed to increase production speed and reduce the required manual labor as for example, U.S. Pat. No. 4,649,248 to Yamaguchi et al. This patent discloses a process where each core is loaded onto a tray for transport through the furnace. As in the batch process, each tray holds the core in a horizontal position such that a conductive magnetizing shaft is vertically disposed in the central opening of the core while the magnetic field is being applied.
The problems with the conventional batch and continuous processes are severalfold. First, each core in the furnace is subjected to an individual magnetic field so that it is difficult to ensure statistical uniformity of material properties as amongst individual cores. Secondly, the cores are annealed in a non-optimal horizontal position with their central openings defining vertical axes. In the field, however, the transformers are oriented such that the cores hang in a vertical position with their central openings defining horizontal axes. It is preferable to anneal the cores in the same position relative to gravity as they are used in operation.
Still further, the Yamaguchi et al process requires multiple current sources, which increases the cost and complexity of the apparatus, and since the cores are annealed while lying on a tray, the heat transfer to the core is uneven. More particularly, heat transfer from the oven to the core occurs by conduction on the side of the core which is in contact with the tray and by convection on the side in contact with the atmosphere of the oven.
It is accordingly an object of the present invention to provide a method and apparatus for magnetic field annealing that allows the cores to be effectively and efficiently annealed in a continuous fashion.
It is another object of the present invention to provide a method and apparatus for magnetic field annealing that maintains the cores in a vertical position while the cores are heated and the magnetic field is being applied, and so as to facilitate the automation of the process.
It is still another object of the present invention to provide a method and apparatus for magnetic field annealing that subjects the individual cores to one common magnetic field during the entire continuous annealing process.
Additional objects of the present invention include the provision of a method and apparatus of the described type which requires only a single current source, and which provides rapid and uniform heat transfer to each core.